2  ( Degree )

Intensity (a.u) (321)

(011) (123)

(152)(013)(161)

(202)(042)(240)(-132)

(002)(200)

(040)

(121)

BiVO4

(BiVO4:In⁾-CNQD

(211)

= FTO

39.0 39.5 40.0 40.5 41.0

(BiVO4:In)-CNQD BiVO4

Intensity (a.u)

2 (Degree)

25 30 35 40 45 50

(002)

CNQD

Intensity (a.u)

2



( Degree )

(a) (b)

Chapter 4 (BiVO4:In)-CNQD

67 Part of this chapter has been published in J. Power Sources, 2020, 477, 229024

The (BiVO4:In)-CNQD photoanode does not show peaks for CNQDs due to very less amount of CNQDs in (BiVO4:In)-CNQD photoanode. Inset in Figure 4.1 (a) shows a shift in PXRD peak at (211) plane of (BiVO4:In)-CNQD photoanode towards lower 2 value in comparison to its pristine counterpart, indicating doping in BiVO4. Figure 4.1 (b) shows the powder X-ray diffraction (PXRD) pattern of CNQDs which confirms the formation of CNQDs.²⁰

4.3.2 Raman Spectroscopy Analysis

Raman analysis shown in Figure 4.2 was performed to further confirm the formation and to know doping in BiVO4 photoanode. Highly intense and weak bands at 870 cm^-1 and 744 cm^-1corresponds to symmetric and asymmetric V-O mode of stretching, respectively. The bands at 386 cm^-1 and 342 cm^-1 correspond to symmetric and asymmetric bending modes of vibration of VO43-tetrahedron. The band at 219 cm^-1 and 129 cm^-1 corresponds to external mode, revealing the structural information of BiVO4.21Raman spectrum of both BiVO4 and BiVO4:In photoanode in Figure 4.2 look identical; however, highly intense peak at ~870 cm^-

1shows a shift towards higher wave number indicates doping in BiVO4.

Figure 4.2. Raman spectra of BiVO4 and BiVO4:In photoanodes where the high intense band (~870) shows a shift towards higher Raman shift (shown in the inset).

100 200 300 400 500 600 700 800 900 1000 BiVO4-In

BiVO4

744 870

386342

219129

Inte ns ity (a .u)

Ramanshift

(

cm-1)

780 810 840 870 900 930 BiVO4

BiVO4-In

Intensity (a.u)

Ramanshift (cm-1₎

TH-2636_166122040

Chapter 4 (BiVO4:In)-CNQD

4.3.3 UV-Visible Spectra Analysis

A UV-visible diffuse reflectance spectrum was used to study the optical properties of various prepared photoanodes. Figure 4.3. (a) shows the absorption curve of (BiVO4:In)- CNQD photoanode, which remains unchanged to that of pristine BiVO4 with an absorption edge at ~530 nm. To further check the formation of CNQDs, UV-visible spectra of CNQDs and bulk g-C3N4 were recorded as shown in Figure 4.3 (b). The blue shift of the CNQDs curve in comparison to bulk g-C3N4 is due to quantum confinement in CNQDs indicating the formation of quantum dots.

Figure 4.3. (a) UV-visible diffuse reflectance spectra of BiVO4 and BiVO4 modified photoanodes. (b) UV-visible spectra of CNQDs and bulk C3N4.

4.3.4 Photoluminescence (PL) Spectra

Further to check the stability of as-synthesized CNQDs Photoluminescence (PL) spectra of CNQDs were recorded (shown in Figure 4.4) just after synthesis and after 7 days of synthesis. There was not much change in PL intensity of CNQDs even after 7 days of synthesis, suggesting hydrothermally synthesized CNQDs using melamine as a precursor is very stable.

300 350 400 450 500 550 600 650 700 In-BiVO4-CNQD BiVO4-CNQD In-BiVO4 BiVO4

Wavelength (nm)

Absorbance (a.u)

300 400 500 600 700 800

Wavelength (nm)

Intensity (a.u)

CNQD

_{C3N4 Bulk}

(b)

(a)

Chapter 4 (BiVO4:In)-CNQD

69 Part of this chapter has been published in J. Power Sources, 2020, 477, 229024

Figure 4.4. Photoluminescence (PL) spectra of CNQDs just after synthesis and after 7 days of storage. Inset in the figure shows CNQD displays strong blue fluorescence when illuminated with UV light.

4.3.5 Morphological and Structural Analysis

Figure 4.5 (a) shows the FESEM image of the seed layer over which crystals of BiVO4

were grown. Figure 4.5 (b) shows the decahedron shaped pristine BiVO4 photoanode grown over the seed layer. Figure 4.5 (c) shows the morphological features of BiVO4:In coupled with g-C3N4 quantum dots.

Figure 4.5 (a) FESEM image of BiVO4 seed layer deposited uniformly over FTO through the spin coating. (b) FESEM image of pristine BiVO4 photoanode. (c) FESEM image of decahedron shaped (BiVO4:In)-CNQD

400 450 500 550 600 650 700

Wavelength (nm)

In tensi ty (a. u )

^CNQD CNQD After 7 days

UV light CNQD

(b) ^(BiVO4)

(c) (d)

Decahedron

(a)

(BiVO₄:In)-CNQD (BiVO₄) Seed layer

(BiVO₄:In)-CNQD

TH-2636_166122040

Chapter 4 (BiVO4:In)-CNQD

photoanode, it includes the representative graphic of decahedron shaped BiVO4:In. (d) Cross-sectional FESEM image of (BiVO4:In)-CNQD photoanode displaying thickness (~1.87m) of the film formed over FTO.

Decahedron shaped BiVO4:In showing facets, which provides higher surface area for the maximum deposition of catalyst for efficient water oxidation. FESEM image of pristine BiVO4 and (BiVO4:In)-CNQD photoanode in Figure 4.5 (b) and Figure 4.5 (c) respectively shows that with indium doping there was no significant change in morphology of BiVO4

photoanode. The cross-sectional FESEM image in Figure 4.5 (d) shows that (BiVO4:In)- CNQD photoanode has a film thickness of ~1.87m formed over FTO.

Figure 4.6 (a) shows a FETEM image of a pristine BiVO4 photoanode. Figure 4.6 (b) shows the FETEM image of well-distributed CNQDs with a uniform particle size of ~ 4 nm.

Figure 4.6 (c) shows the FETEM image of (BiVO4:In)-CNQD where CNQDs are well embedded on the surface of (BiVO4:In),making good electronic interaction between BiVO4:In and CNQDs which is distinguishable from pristine BiVO4 photoanode as shown in Figure 4.6 (a). The HRTEM image of (BiVO4:In)-CNQD in Figure 4.6 (d) shows different lattice spacing of 0.29 nm and 0.33 nm which corresponds to (040) and (002) crystal plane of BiVO4 and CNQDs, respectively, confirming loading of CNQDs over indium doped BiVO4:In.^22,23

(b)

(c)

(BiVO₄:In) (CNQD)

0.33 nm (CNQD) (002)

(BiVO₄:In) 0.29 nm (040)

(d)

(a) CNQD

(BiVO₄:In)-CNQD BiVO₄

Chapter 4 (BiVO4:In)-CNQD

71 Part of this chapter has been published in J. Power Sources, 2020, 477, 229024

Figure 4.6 (a) FETEM image of pristine BiVO4 photoanode. (b) FETEM image of CNQDs. (c) FETEM image of (BiVO4:In)-CNQD photoanode. (d) HRTEM image of (BiVO4:In)-CNQD showing (040) and (002) crystal plane of BiVO4 and CNQDs, respectively.

To further confirm the presence and uniform distribution of all the elements present, FETEM-EDX elemental mapping of (BiVO4:In)-CNQD photoanode (Figure 4.7 (a-f)) was conducted. Elemental mapping shows that the elements Bi, V, O, In, C and N are uniformly distributed throughout the (BiVO4:In)-CNQD photoanode.

Figure 4.7 (a-f) shows uniform distribution of Bi, V, O, In, C, and N respectively in (BiVO4:In)-CNQD photoanode.

4.3.6 FTIR Spectra Analysis

To further confirm formation and interaction between CNQD and BiVO4:In in (BiVO4:In)-CNQD, FTIR of pristine BiVO4, CNQDs and (BiVO4:In)-CNQD were performed as shown in Figure 4.8. A peak at 477cm^-1 corresponds to symmetric bending of VO43- while peaks at 816 cm^-1and 735 cm^-1 corresponds to symmetric and asymmetric stretching of VO43-

which are clearly visible in BiVO4. Inset in Figure 4.8 shows FTIR spectra of CNQDs and (BiVO4:In)-CNQD. Peaks at 1541 cm^-1, 1470 cm^-1, 1320 cm^-1, 1262 cm^-1 correspond to symmetric stretching vibration mode of CN bond and CN heterocycles for CNQDs.^2,3 In

O

1 m In

1 m

C

1 m

N

1 m V

1 m Bi

1 m (a)

(e) (f) (d)

(c) (b)

TH-2636_166122040

Chapter 4 (BiVO4:In)-CNQD

(BiVO4:In)-CNQD photoanode, there is a shift in CNQDs peaks from 1471 cm^-1 to 1464 cm^-1 towards lower frequency, which may be due to the weakening of CN bond indicating good interaction between CNQDs and BiVO4:In.

Figure 4.8. FTIR spectra of pristine BiVO4, CNQDs and BiVO4 modified photoanodes. The inset shows a shift in the FTIR peak of CNQDs in (BiVO4:In)-CNQD.

4.3.7 X-Ray Photoelectron Spectroscopy (XPS)

To find the electronic structural changes by virtue of chemical state and elemental composition of as-synthesized BiVO4 and (BiVO4:In)-CNQD photoanode, XPS analysis was performed. The presence of corresponding elements was confirmed by the XPS survey spectra, as shown in Figure 4.9 (a). Bi 4f core-level spectra of BiVO4 and (BiVO4:In)-CNQD photoanodes are shown in Figure 4.9 (b). Peaks at a binding energy of 159.08 eV and 164.37 eV correspond to Bi- 4f7/2 and 4f5/2 respectively of pristine BiVO4,while for (BiVO4:In)- CNQD photoanode, Bi- 4f7/2 and 4f5/2 peaks appear at binding energy 158.77 eV and 164.07 eV. The presence of Bi- 4f7/2 and 4f5/2 peaks confirms the presence of Bi³⁺ in BiVO4 and (BiVO4:In)-CNQD photoanodes.²⁴ A shift in Bi - 4f7/2 and 4f5/2 peaks of (BiVO4:In)-CNQD compared to pristine BiVO4 photoanode is indicative of a change of electronic structure around Bi,suggests that the doping in the A-site of BiVO4 in (BiVO4:In)-CNQD photoanode. Figure

4000 3500 3000 2500 2000 1500 1000 500 477

816 735 BiVO4

477 CNQD

735 816